A Study to Investigate the Radiological Onset of Action After Treatment Initiation With Subcutaneous (SC) Natalizumab in Participants With Relapsing-Remitting Multiple Sclerosis (RRMS)

Cumulative Number of CUALs Through Weeks 4, 8, and 12

CUALs will be calculated as the sum of the number of Gd-enhancing lesions and new or enlarging T2 hyperintense lesions that are non-enhancing on post-gadolinium T1w scans.

Time frame: Weeks 4, 8, and 12

Absolute Number of CUALs at Weeks 4, 8, 12, and 24

CUALs will be calculated as the sum of the number of Gd-enhancing lesions and new or enlarging T2 hyperintense lesions that are non-enhancing on post-gadolinium T1w scans.

Time frame: Weeks 4, 8, 12, and 24

Mean Change From Baseline of CUALs at Weeks 4, 8, 12, and 24

CUALs will be calculated as the sum of the number of Gd-enhancing lesions and new or enlarging T2 hyperintense lesions that are non-enhancing on post-gadolinium T1w scans.

Time frame: Baseline, Weeks 4, 8, 12, and 24

Cumulative Number of New Gd-Enhancing Lesions Through Weeks 4, 8, 12, and 24

Cumulative number will be calculated as the sum of the number of Gd-enhancing lesions through Weeks 4, 8, 12, and 24.

Time frame: Weeks 4, 8, 12, and 24

Absolute Number of New Gd-Enhancing Lesions at Weeks 4, 8, 12, and 24

Time frame: Weeks 4, 8, 12, and 24

Absolute Number of Persisting Gd-Enhancing Lesions at Weeks 4, 8, 12, and 24

Time frame: Weeks 4, 8, 12, and 24

Absolute Number of Any (New or Persisting) Gd-Enhancing Lesions at Weeks 4, 8, 12, and 24

Time frame: Weeks 4, 8, 12, and 24

Change From Baseline of Any (New or Persisting) Gd-Enhancing Lesions at Weeks 4, 8, 12, and 24

Time frame: Baseline, Weeks 4, 8, 12, and 24

Cumulative Number of New or Enlarging T2 Lesions at Weeks 4, 8, 12, and 24

Cumulative number will be calculated as the sum of the number of new or enlarging T2 hyperintense lesions through Weeks 4, 8, 12, and 24.

Time frame: Weeks 4, 8, 12, and 24

Absolute Number of New or Enlarging T2 Lesions at Weeks 4, 8, 12, and 24

Time frame: Weeks 4, 8, 12, and 24

Change From Baseline of New or Enlarging T2 Lesions at Weeks 4, 8, 12, and 24

Time frame: Baseline, Weeks 4, 8, 12, and 24

Annualized Relapse Rate

Multiple sclerosis (MS) relapse is defined as the onset of new or recurrent neurological symptoms lasting at least 24 hours, accompanied by new objective abnormalities on a neurological examination, and not explained solely by non-MS processes such as fever, infection, severe stress, or drug toxicity. ARR is defined as the total number of relapses divided by the total participant-time at risk of relapse.

Time frame: Week 24

Time to First Relapse

MS relapse is defined as the onset of new or recurrent neurological symptoms lasting at least 24 hours, accompanied by new objective abnormalities on a neurological examination, and not explained solely by non-MS processes such as fever, infection, severe stress, or drug toxicity. Time to first MS relapse will be calculated as the date from first study drug administration through the date of the first relapse, if applicable.

Time frame: Up to Week 24

Number of Participants With Expanded Disability Status Scale (EDSS) Improvement and Stable Disease and Worsening at Weeks 12, and 24

The EDSS measures the disability status of people with multiple sclerosis on a scale that ranges from 0 to 10. The first levels 1.0 to 4.5 refer to people with a high degree of ambulatory ability and the subsequent levels 5.0 to 9.5 refer to the loss of ambulatory ability. The range of main categories include (0) = normal neurologic exam; to (5) = ambulatory without aid or rest for 200 meters; disability severe enough to impair full daily activities; to (10) = death due to MS. Stable disease is defined as +/- 0.5 change of EDSS. Worsening is \> 0.5 increase of EDSS.

Time frame: Baseline, Weeks 12, and 24

Trough Serum Natalizumab Concentration (Ctrough)

Time frame: Pre dose on Baseline, Weeks 4, 8, 12, and 24

Trough alpha 4 (α4) Integrin Saturation

Time frame: Pre dose on Baseline, Weeks 4, 8, 12, and 24

Change From Baseline in Lymphocyte Subsets Count

Lymphocyte subsets include T cells, B cells and natural killer cells (cluster of differentiate 4 \[CD4\], CD8, CD19, and CD56).

Time frame: Baseline up to Week 24

Change From Baseline in Anti-Natalizumab Antibodies

Time frame: Pre dose on Baseline, Weeks 12, and 24

Persistence of Anti-Natalizumab Antibodies

Re-test will be done for antibodies after 6 weeks of first positive result.

Time frame: Re-test after 6 weeks of first positive result (up to Week 24)

Number of Participants With Adverse Events (AEs) and Serious Adverse Events (SAEs)

An AE is any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease temporally associated with the use of a medicinal (investigational) product, whether or not related to the medicinal (investigational) product. SAE is any untoward medical occurrence that at any dose results in death, in the view of the investigator, places the participant at immediate risk of death (a life-threatening event), requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, results in a congenital anomaly/birth defect or is a medically important event.

Time frame: Up to Week 24